I like simple installations with a single battery but with mechanical retracts the possibility of having a stalled retract servo is quite high and this will flatten the battery quickly so using a seperate battery for the retract servo is a good idea.

Here is one way of doing this.

You need an additional switch and a 'Y' lead, the latter modified to allow the second battery to provide power exclusively to the retract servo. To this end the red (positive) lead is either cut through or removed from the receiver end of the plug, thus ensuring that only the secondary battery pack provides power to the servo, eliminating any possibility of a stalled retract servo flattening the Rx pack. The 'Y' lead is then connected to the retract channel of the Rx, with the second battery being plugged into one of the sockets of the 'Y' lead via a switch. The retract servo is plugged into the leads remaining socket.